Noncorrosive aluminum alloy



Patented 7, p 1930 mm; mama-Banana,- or

- Lunar oonromrromor NEW roax,

noncomsrvn rm: Io Drawing. Application filed February 84, 1926, Serial Io. 90,482, and in Switzerland December 17,

The present invention relates to aluminum alloys resistant to the action of corrosive agents and particularly to sea-water and to the production of such alloys. g

It is well known that the alloys of aluminum and magnesium hitherto available are very deficient in resistance to the corroslve action of air, water and weather, and for this reason they have not been put to the extensive use which their advantageous mechanical properties would otherwise have indicated.

It has now been found, however, that cer tain aluminum-magnesium alloys may be made, suitable for uses'where corrosion-resisting properties are required, by means of suitabletreatment hereinafter more fully described.

The treatment of the magnesium-aluminum alloys to render the corrosion-resistant comprises-a suitable thermal treatmentv advantageously in conjunction with suitable additions to the alloy.

The effect of the thermal treatment of the invention in increasing the resistance of the th alloys to corrosionis related to the fact that, g-ty t l b i th b t f d in the presence of corrosive agents, otential differences arise between the lssimilar crystals of heterogeneous alloys, that is to say, of alloys made up, in the solid state, of crystals of more than one physical type. These otential difl'erences give'rise to intercrystalline electrolytic action and greatly accelerate corrosion.

Scientific research has shown that at ordinary telnperatures aluminum can hold magnesium in solid solution to the extent of about 10%. -However, the casting of an aluminum alloy containing up to 10% magnesium does not result in castings of homogeneous crystal structure as might be expected. On the con trary, clue to the high crystallization speed of the intermetallic'compound Al Mg (containing over 37% magnesium), and to the low rate of diffusion, segregation occurs and structures of poor resistance to corrosion are formed. The high rate of crystallization results in the first crystals'bcing of higher magnesium content than can be held in solid solution at ordinary temperatures. Since these first ,B-type crystals are richer in magnesium p 7 umrao mm- PArENr OFFICE,

NECKABSULI, GERMANY, 'ASSIGNOB TO AMERICAN N. Y A CORPORATION Oil NEW YORK above described is high, for a time sufiicient to permit of substantially complete diffusion. In .this way there is produced a magnesium-aluminum alloy free from intercrystalline potential differences and of surprising resistance to corrosive agents. v 1

For instance, magnesium-aluminum alloys containing 10% of magnesium may be held for about an hour at a temperature of about 420 C. Under magnesium in the e magnesium-poor a-type crystals, the

1nto -type crystals so that the resulting alloy conslsts entirely of crystals of the same these conditions the excess ,B-type crystals diffuses into crystal-structure between which there is substantially no potential difference.

A similar result is caused to cool down from a temperature of about 420 C. over a period of 5 to 8 hours.

A 5 to 20% valuable etching reagent for the determination of the presence of the diverse crystal types and for following the diffusion and omogenizing' of the structure of the alloy. ;-Of further value in increasing the reslstance tocorrosion of alloys of this type is the addition to the alloys of substances which obtained if the casting is solution of chromic acid is a tend to produce protective oxide coatings or I films on, the surface of with the air. I

It has been found that antimony, bismuth and cadmium answer the requirements above set forth and form highly suit-able addition agents, either alone or in admixture, in

the alloy in contact aluminum alloys of ;the type under consideration.

However, the very .low solubility of the above-named addition substances prevents properties of the alloys, without giving rise to lntercrystallinepotential difierences in the alloys, but also increases the amount of the corrosion-reducing substances, such as antimony, bismuth, and cadmium, which may be added without causing a substantial intercrystalline potential difference in the alloys.

An alloy composition within the scope of the present invention is:

Magnesium 3 to 6% Manganese 1 to 4% Antimony up to 1% Remainder aluminum.

When the chromium is added to the alloys it should not be in amounts greater than 2%,

and preferably less than 1%, because of its efiect in raising the solidification temperature of the alloys. For instance, with 2% chromium solidification begins at about 8009, with 3% chromium at about 900, and with 4% chromium at about 1000.

It is notnecessary that all ofthe above mentioned types of addition substances be present in the alloys, as, for instance, magnesium-manganese-aluminum alloys, -when produced according to the teachings of the present invention, afford very considerable advantages over hitherto known aluminum alloys. The same is true when, instead of manganese or magnesium, small quantities of antimony, bismuth, or cadmium enter into the composition of the'alloy.

Further alloy compositions included within the scope of the present invention comprise:

1. Magnesium; 3 to 6% Manganese 1 to 4% Chromium 0.5%

. Remainder aluminum 2. Magnesium 3 to 6% Cadmium 1.5% Remainder aluininu 3. Manganese 2% Chromium ).5%

Antimony 3% Remainder aluminum In the following claims the term metal of the iron, group is intended to include manganese which, although it is not included in group 8 of the Periodic System, performs the same function in this invention as the y metals of the iron group proper.

I claim:

1. Amprocess of making corrosion-resistant alloys, which comprises subjecting an alloy of aluminum and magnesium containing not more than about 10 per cent of magnesium to an elevated tem erature below its melting point until the al oy contains substantially only one crystal type.

, 2. A process of making corrosion-resistant alloys, which comprises subjecting an alloy of aluminum and magnesium containing not more than about 10 per cent of magnesium to a temperature of about 420 C. until the alloy contains substantially only one crystal type.

3. process of making corrosion-resistant alloys, which comprises subjecting an alloy of aluminum and magnesium containing not more than about 10 per cent of magnesium at a temperature until the ,8 crystals have disappeared.

4. corrosion-resistant alloy of aluminum and magnesium, the magnesium content of which does not substantially exceedthe solid solubility of magnesium in aluminum, said alloy being free crystals.

5. A corrosion-resistant alloy of aluminum and magnesium containing not more than about 10% of magnesium and being free from magnesium-rich ,B-type crystals.

6. A corrosion resistant alloy of aluminum and magnesium, the magnesium-content of which does not substantially exceed the solid solubility of ma alloy being free rom magnesium-rich ,B-type crystals and containing at least one metal of the group comprising antimony, bismuth and cadmium.

7. A corrosion-resistant alloy of aluminum and magnesium, the magnesium content of which does not substantially exceed the solid solubility of ma nesium in aluminum, said,

alloy being free rom magnesium-rich ,B-type crystals and containing at least one metal of the group comprising antimony, bismuth and cadmium, and at least one metal of. the iron group.

8. A corrosion-resistant alloy of aluminum and magnesium, the magnesium content of which does not substantially exceed the solid solubility of magnesium in aluminum, said 10. A corrosion-resistant ,alloy' of aluminum and magnesium, the magnesium content esium in aluminum, said rom magnesium-rich fl-type ROLAND STERNER-RAINER. 

